An Irish man is one of three people who have proven they can match the brains of Britain's intelligence and security organisation by cracking a puzzle set by its boss on a Christmas card.

More than 600,000 people tried to solve the Government Communications Headquarters (GCHQ) puzzle, with 30,000 making it to the final round of the challenge.

Nobody figured the whole thing out, but three got close - and one of them was David McBryan who is originally from Dublin.

The 41-year-old - who now lives in Edinburgh - admits it was tough:

"In total I spent probably the best part of three days on it. 

The difficult bit is definitely the last stage, because you don't know when you are finished, you don't know how many answers they are looking for, you don't know what form they should take."

Problem-solving and terrorism

This man - who cannot be identified - is one of those who came up with the puzzle.

He says tackling it shares some similarities to that of trying to catch terrorists:

"You are working, perhaps with some ambiguity, some difficult problems, some difficult data - figuring out what to do, figuring out if it is working.

So, the same sort of problem-solving skills, applied in a different context."